The instant invention is directed to an improved battery separator membrane suitable for use in alkaline battery systems, such as nickel-zinc systems, which is capable of exhibiting an unexpected high degree of dendrite inhibition, electrical conductivity, stability to alkali, and ability to form very thin membranes.
Alkaline battery systems, because of their high energy density, have great potential for replacing the more conventional lead-acid battery system in a number of terrestrial applications. However, extending the cycle life of such batteries beyond that presently attainable and reducing the cost of all the components are required criterias which must be met to make the alkaline battery an effective energy source.
One of the recognized key components in extending the life and efficiency of the battery is its separator. The separator is a membrane located between the plates of opposite polarity to prevent contact between the plates while freely permitting electrolytic conduction. Contact between plates may be due to imperfections in the plate structure or due to warping or wrinkling of the plate during use. Such macro deformations are readily inhibited by any type of sheet material which is coextensive with that of the plates. Contact may also occur due to the formation of dendrites or localized needle like growths on an electrode, such as zinc dendrites formed on a zinc electrode in an alkaline nickel-zinc battery system. These dendrites bridge the gap between electrodes of opposite polarity either by puncturing the separator membrane located in the gap, or by passing through the pores of the separator. The high degree of solubility of zinc oxide in alkaline electrolytes normally permits extensive loss of active material from the negative electrode through deposition of the zinc oxide in the separator pores and onto the positive electrode. These factors cause shorting out of the battery system and significantly reduce its effective life. The ability to produce a separator membrane which can effectively act as a dendristatic diaphragm is a required criteria for forming an effective battery system.
Further, a separator suitable for use in forming a highly effective alkaline battery system must be capable of exhibiting a high degree of electrical conductivity. Stated another way, an effective separator membrane must exhibit a low electrical resistance and good wetting properties.
U.S. Pat. No. 3,351,495 discloses battery separators for use in both acid and alkaline battery systems formed from very high molecular weight polyolefin compounded with a plasticizer and an inert filler. The reference further teaches that, in alkaline battery separators, filler having relatively low surface areas, e.e., one square meter per gram or less, are satisfactorily employed. Battery separators formed in accordance with the general procedure and materials disclosed in this patent exhibit a high degree of electrical resistance and poor wetting properties. These separators, therefore, do little to enhance the efficiency and effectiveness of the resultant battery systems.
U.S. Pat. No. 4,024,323 is directed to a variation of the 3,351,495 battery separator which aids in processability. The resultant separator has similar defects.
A battery separator which is capable of increasing the efficiency of a battery system and cause it to have a high energy density is highly desired, especially with respect to alkaline battery systems. It is generally agreed that such separators should be (a) resistant to degradation by the alkaline electrolyte and by oxidation due to nascent oxygen, (b) be very thin, (c) exhibit a high degree of inhibition to dendrite formation and growth, and (d) exhibit a high degree of electrical conductivity. The first two elements and the last two elements are each thought to be counter productive with respect to each other. For example, very thin sheets have a high surface area to volume ratio and are, therefore, more susceptible to attack by the strong alkaline electrolyte solution and to oxidation. With respect to the latter two criterias, it is known that separator membranes which are nonporous normally exhibit a high degree of inhibition to dendrite formation, but have low electrical conductivity. Microporous separators, that is those which have discrete pores usually in the form of a tortuous network, have a high degree of electrolyte permeability but they lack the ability to inhibit dendritic shorting.